5894307-UIM-B-0920
Johnson Controls Ducted Systems
21
Ambient Combustion Air Supply
This type of installation draws the air required for combustion from
within the space surrounding the appliance and from areas or rooms
adjacent to the space surrounding the appliance. This can be from
within the space in a non-confined location or it can be brought into the
furnace area from outdoors through permanent openings or ducts. It is
not piped directly into the furnace. Provide a single, correctly sized pipe
from the furnace vent connector to the outdoors. It is best practice to
attach the supplied intake coupling and 18 in. of pipe to the furnace to
prevent accidental blockage of the combustion air intake.
An
unconfined space
is not less than 50 cu. ft (1.42 m
3
) per 1,000
BTU/hr (0.293 kW/h) input rating for all of the appliances installed in
that area.
Rooms communicating directly with the space containing the appli-
ances are considered part of the unconfined space, if doors are fur-
nished with openings or louvers.
A
confined space
is an area with less than 50 cu. ft (1.42 m
3
) per
1,000 BTU/hr (0.293 kW/h) input rating for all of the appliances installed
in that area. The following must be considered to obtain proper air for
combustion and ventilation in confined spaces.
Dampers, Louvers, and Grilles
The blocking effects of louvers, grilles, and screens must be given con-
sideration in calculating free area. If the free area of a specific louver or
grille is not known, see Table 9 to estimate free area.
* Do not use less than 1/4 in. (6.4 mm) mesh.
+ Free area of louvers and grille varies widely. Follow the louver or grille manufac-
turer’s instructions.
• The free area of a supply air opening must be calculated by sub-
tracting the blockage area of all fixed louvers grilles or screens
from the gross area of the opening.
• Apertures in a fixed louver, grille, or screen must have no dimen-
sion smaller than 1/4 in. (6.4 mm).
• A manually operated damper or manually adjustable louvers are
not permitted for use.
• An automatically operated damper or automatically adjustable
louvers must be interlocked so the main burner can not operate
unless either the damper or the louver is in the fully open position.
Figure 27:
Combustion Airflow Path Through the Furnace Casing
WARNING
This type of installation requires that the supply air to the appliance(s)
be of a sufficient amount to support all of the appliance(s) in the area.
Operation of a mechanical exhaust, such as an exhaust fan, kitchen
ventilation system, clothes dryer or fireplace may create conditions
requiring special attention to avoid unsatisfactory operation of gas
appliances. A venting problem or a lack of supply air will result in a
hazardous condition, which can cause the appliance to soot and gen-
erate dangerous levels of CARBON MONOXIDE, which can lead to
serious injury, property damage, and/or death.
COMBUSTION AIR
VENT PIPE
CLAMPS INTO
OUTLET OF
VENT COUPLING
RAIN GUTTER
A1131-001
!
Table 9:
Estimated Free Area
Wood or Metal
Louvers or Grilles
Wood 20–25%*
Metal 60–70% *
1/4 in. (6.4 mm)
mesh or larger 100%
WARNING
When a Category I furnace is removed or replaced, the original vent-
ing system may no longer be correctly sized to properly vent the
attached appliances.
An improperly sized vent system can cause CARBON MONOXIDE to
spill into the living space causing personal injury and/or death.
Table 10:
Unconfined Space Minimum Area
BTUH Input Rating Minimum Free Area Required for Each Opening
60,000
60 in
2
(387 cm
2
)
80,000
80 in
2
(516 cm
2
)
100,000
100 in
2
(645 cm
2
)
Table 11:
Free Area
BTUH Input
Rating
Minimum Free Area Required for Each Opening
Horizontal Duct
(2,000 BTUH)
Vertical Duct or
Opening to Outside
(4,000 BTUH)
Round Duct
(4,000 BTUH)
60,000
30 in
2
(193 cm
2
)
15 in
2
(97 cm
2
)
5 in. (13 cm)
80,000
40 in
2
(258 cm
2
)
20 in
2
(129 cm
2
)
5 in. (13 cm)
100,000
50 in
2
(322 cm
2
)
25 in
2
(161 cm
2
)
6 in. (15 cm)
EXAMPLE: Determining Free Area.
Appliance
1 Appliance
2 Total Input
100,000 + 30,000 = (130,000
4,000) = 32.5 sq. in. vertical
Appliance
1 Appliance
2 Total Input
100,000 + 30,000 = (130,000
2,000) = 65 sq. in. horizontal
!
